Maxwell's equations-based dynamic laser-tissue interaction model

Abstract

Since its invention in the early 1960s, the laser has been used as a tool for surgical, therapeutic, and diagnostic purposes. To achieve maximum effectiveness with the greatest margin of safety it is important to understand the mechanisms of light propagation through tissue and how that light affects living cells. Lasers with novel output characteristics for medical and military applications are too often implemented prior to proper evaluation with respect to tissue optical properties and human safety. Therefore, advances in computational models that describe light propagation and the cellular responses to laser exposure, without the use of animal models, are of considerable interest. Here, a physics-based laser-tissue interaction model was developed to predict the dynamic changes in the spatial and temporal temperature rise during laser exposure to biological tissues. Unlike conventional models, the new approach is grounded on the rigorous electromagnetic theory that accounts for wave interference, polarization, and nonlinearity in propagation using a Maxwell's equations-based technique.

Keywords: Finite element; Laser damage; Laser–tissue interaction; Photoacoustic.

Fig. 1

A schematic diagram of the (a) 2D FE domain in Cartesian coordinates system (b) linear triangular element.

Fig. 2

A schematic diagram of the skin model.

Fig. 3

Maxwell model transient center temperature response for (a) 1540 nm wavelength for 0.24 s, 0.1 s, 0.045 s, and 0.025 s exposure durations, 1-mm spot diameter (b) 2000 nm wavelength for 4 s, 2 s, 1 s, 0.5 s, 0.25 s, and 0.1 s exposure duration, 1.17 mm spot diameters.

Fig. 4

Peak temperature as a function of radiant exposure and damage thresholds for 694-nm wavelength, 1- and 2-mm diameters, and 40-ns pulse duration.

Fig. 5

Maxwell model transient center temperature response for (a) 193 nm wavelength for, 13 ns exposure duration, 40 mJ/cm², 90 mJ/cm², and 135 mJ/cm² radiant exposures (b) Temperature profile at the end of a 193 nm wavelength, 40 mJ/cm² radiant exposure, and 13-ns pulse duration.

Fig. 6

(a) Transient center temperature response for 2940 nm wavelength for 0.3 s exposure duration, and a 1.0 mm spot diameter. (b) Induced pressure profile for static simulation (c) Induced pressure profile for dynamic simulation

Fig. 7

Illustration of threshold temperature at the center and boundary of cell death.